As practiced commercially, fused silica optical members such as lenses, prisms, filters, photomasks, reflectors, etalon plates and windows, have been manufactured from bulk pieces of fused silica made in large production furnaces. Bulk pieces of fused silica manufactured in large production furnaces are known in the art as boules or ingots. Blanks are cut from boules or ingots, and finished optical members are manufactured from glass blanks, utilizing manufacturing steps that may include, but are not limited to, cutting, polishing, and/or coating pieces of glass from a blank. Many of these optical members are used in various apparatus employed in environments where they are exposed to ultraviolet light having a wavelength of about 360 nm or less, for example, an excimer laser beam or some other ultraviolet laser beam. The optical members are incorporated into a variety of instruments, including lithographic laser exposure equipment for producing highly integrated circuits, laser fabrication equipment, medical equipment, nuclear fusion equipment, or some other apparatus which uses a high-power ultraviolet laser beam.
As the photon energy, pulse energy and pulse rate of lasers increase, the optical members which are used in conjunction with such lasers are exposed to increased levels of energy. Fused silica has become widely used as the material of choice for optical members in such laser-based optical systems due to their excellent optical properties and resistance to laser induced damage.
Laser technology has advanced into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength lasers operating in the UV and deep UV (DUV) and vacuum UV wavelength ranges, which include, but are not limited to, lasers operating at about 248 nm, 193 nm, 157 nm and even shorter wavelengths. Excimer laser systems are popular in microlithography applications, and the shortened wavelengths allow for increased feature resolution and thus line densities in the manufacturing of integrated circuits and microchips, which enables the manufacture of circuits having decreased feature sizes. A direct physical consequence of shorter wavelengths (higher frequencies) is higher photon energies. In such optical systems, fused silica optics are exposed to high irradiation levels for prolonged periods of time, and this may result in the degradation of the optical properties of the optical members.
It is known that such laser induced degradation adversely affects the optical properties and performance of the fused silica optics by decreasing light transmission levels, discoloring the glass, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Over the years, many methods have been suggested for improving the optical damage resistance of fused silica glass. It has been generally known that high purity fused silica prepared by such methods as flame hydrolysis, CVD-soot remelting process, plasma CVD process, electrical fusing of quartz crystal powder, and other methods, is susceptible to laser damage to various degrees.
A common suggestion has been to increase the OH content of such glass to a high level. For example, Escher, G. C., KrF Laser Induced Color Centers In Commercial Fused Silicas, SPIE Vol. 998, Excimer Beam Applications, pp. 30-37 (1988), confirms that defect generation rate is dependent upon the fused silica OH content, and that “wet” silica is the material of choice for KrF applications. Specifically, they note that high OH content silica is more damage resistant than low OH silica.
U.S. Pat. No. 5,086,352 and the related U.S. Pat. No. 5,325,230 has also disclosed that the ability to resist optical deterioration from exposure to a short wavelength ultraviolet laser beam depends on the OH group content in the presence of hydrogen. Specifically, these references show that for high purity silica glass having low OH content, KrF excimer laser durability is poor. Thus, they suggest an OH content of at least 50 ppm. Similarly, Yamagata, S., Improvement of Excimer Laser Durability of Silica Glass, Transactions of the Materials Research Society of Japan, Vol. 8, pp. 82-96 (1992), discloses the effect of dissolved hydrogen on fluorescence emission behavior and the degradation of transmission under irradiation of KrF excimer laser ray for high purity silica glass containing OH groups up to 750 ppm by weight such as those synthesized from high purity silicon tetrachloride by the oxygen flame hydrolysis method.
Others have also suggested methods of increasing the optical durability of fused silica. For example, Faile, S. P., and Roy, D. M., Mechanism of Color Center Destruction in Hydrogen Impregnated Radiation Resistant Glasses, Materials Research Bull., Vol. 5, pp. 385-390 (1970), have disclosed that hydrogen-impregnated glasses tend to resist gamma ray-induced radiation damage. Japanese Patent Abstract 40-10228 discloses a process by which a quartz glass article made by melting is heated at about 400 to 1000° C. in an atmosphere containing hydrogen to prevent colorization due to the influence of ionizing radiation (solarization). Similarly, Japanese Patent Abstract 39-23850 discloses that the transmittance of UV light by silica glass can be improved by heat treating the glass in a hydrogen atmosphere at 950 to 1400° C. followed by heat treatment in an oxygen atmosphere at the same temperature range.
Shelby, J. E., Radiation Effects in Hydrogen-impregnated Vitreous Silica, J. Applied Physics, Vol. 50, No. 5, pp. 3702-06 (1979), suggests that irradiation of hydrogen-impregnated vitreous silica suppresses the formation of optical defects, but that hydrogen impregnation also results in the formation of large quantities of bound hydroxyl and hydride, and also results in a change in density of the glass.
Recently, U.S. Pat. No. 5,410,428 has disclosed a method of preventing induced optical degradation by a complicated combination of treatment processes and compositional manipulations of the fused silica members to achieve a particular hydrogen concentration and refractive index, in order to improve resistance to UV laser light degradation. It is suggested that under such UV irradiation some chemical bonds between silicon and oxygen in the network structure of the fused silica is generally broken and then rejoins with other structures resulting in an increased local density and an increased local refractive index of the fused silica at the target area.
More recently, U.S. Pat. No. 5,616,159 to Araujo et al. disclosed a high purity fused silica having high resistance to optical damage up to 107 pulses (350 mJ/cm2/pulse) at the laser wavelength of 248 nm and a method for making such glass. The composition disclosed in Araujo et al. comprises at least 50 ppm OH and has a concentration of H2 greater than 1×1018 molecules/cm3.
It has been reported that when silica glass is exposed to non-polarized or circularly polarized UV laser beam, usually in the peripheral area of the exposure light beam, additional birefringence (induced edge birefringence) is generated due to strain caused by laser damage, but in the center area of the light beam, there is usually negligible induced birefringence. Recently, a new phenomenon of laser damage to silica material has been observed: when the silica glass is exposed to linearly polarized deep UV laser beam, in addition to the induced edge birefringence, additional birefringence is induced in the center of the exposed area of the glass (“polarization-induced birefringence”). The induced birefringence, especially polarization-induced birefringence, is of particular concern to immersion lithography systems where a liquid fills the gap between the last lens element and the wafer in order to enlarge the numerical aperture of the lens system. In such immersion lithography systems, the polarization state of the UV radiation needs to be controlled, desirably linearly polarized. The induced birefringence in the glass alters the polarization state of the UV radiation, causing reduction of phase contrast and system resolution. Therefore, for deep UV and vacuum UV immersion lithographic systems, it is highly desirable that the glass material used in making the lens elements has low induced birefringence damage, especially a low polarization-induced birefringence, when exposed to linearly or elliptically polarized UV radiation, in addition to low laser-induced wave-front distortion (“LIWFD”) and high transmission.
C. K. Van Peski et al. reports in Journal of Non-Crystalline Solids 265, 285 (2000) the experimental observation of the polarization-induced birefringence effect within a SEMATECH study in which fused silica of several suppliers was exposed and measured. The publication does not disclose any details about the samples other than the measured damage. N. F. Borrelli et al., Applied Physics Letters 80 (2), 219 (2002) reports observation of the effect in different types of fused silica made via the so-called direct glass process. Here, polarization-induced birefringence was shown to occur in samples exhibiting retarded wavefront in the damaged area (then called compaction) as well as in samples exhibiting advanced wavefront in the damaged area (then called expansion). An explanation of polarization-induced birefringence in terms of anisotropic density change is introduced. European patent application EP 1340722A1 discloses polarization-induced birefringence levels for several glasses of different compositions made by using the soot-to-glass process. Those glasses were loaded with molecular hydrogen during consolidation of the soot. These references do not teach high purity synthetic silica material with a low level of polarization-induced birefringence and method of making the same.
Therefore, there exists a need for a synthetic silica material having a low level of polarization-induced birefringence and method of making the same. The present invention satisfies this need.